Weaving Calculations: Loom Production, Reed Count, GSM and Yarn Requirement

The main weaving calculations are loom production (metres per hour = loom rpm × 60 × efficiency ÷ (PPI × 39.37)), total warp ends (EPI × cloth width in inches), reed width and reed count (reed width = cloth width × (1 + weft crimp % ÷ 100); dents per inch = total ends ÷ (reed width × ends per dent), and the Stockport count is twice that), fabric weight (GSM = 23.25 × [EPI × (1 + warp crimp) ÷ warp Ne + PPI × (1 + weft crimp) ÷ weft Ne]), yarn requirement (warp kg = ends × fabric length in metres × (1 + warp crimp) × tex ÷ 1,000,000, and the weft the same way from picks per metre and reed width), cover factor (threads per inch ÷ √Ne, where 28 means full cover) and crimp ((straightened yarn length − fabric length) ÷ fabric length × 100). This page works one example fabric through every formula, from the reed to the loom, so each answer can be checked against the others, and links each step to the site’s calculator for it.

Last verified: 8 October 2026. The example fabric throughout is a 63-inch cotton cloth with 120 ends and 80 picks per inch in 40s Ne warp and weft, 6% warp crimp and 4% weft crimp. Crimp and efficiency go into the formulas as fractions (6% = 0.06). Sources are listed at the end.

Weaving formulas at a glance

What you want	Formula	Example fabric
Total warp ends	EPI × cloth width (inches), plus selvedge ends	120 × 63 = 7,560 ends
Reed width	cloth width × (1 + weft crimp)	63 × 1.04 = 65.52 inches
Reed count	dents per inch = total ends ÷ (reed width × ends per dent); Stockport = dents per 2 inches	57.7 dents per inch = 115 Stockport
Fabric weight	GSM = 23.25 × [EPI × (1 + warp crimp) ÷ warp Ne + PPI × (1 + weft crimp) ÷ weft Ne]	122.3 g/m²
Cover factor	K = threads per inch ÷ √Ne; cloth K = K1 + K2 − K1 × K2 ÷ 28	23.05, or 82% of full cover
Crimp	(straightened yarn length − fabric length) ÷ fabric length × 100	106 cm of yarn from 100 cm of cloth = 6%
Warp yarn	kg = ends × fabric length (m) × (1 + warp crimp) × tex ÷ 1,000,000	118.3 kg per 1,000 m of cloth
Weft yarn	kg = PPI × 39.37 × fabric length (m) × reed width (m) × tex ÷ 1,000,000	77.4 kg per 1,000 m of cloth
Loom production	m/h = rpm × 60 × efficiency ÷ (PPI × 39.37)	9.72 m/h at 600 rpm and 85%
Loom efficiency	picks inserted ÷ (rpm × minutes run) × 100	260,000 picks in 8 hours at 600 rpm = 90.3%
Weft insertion rate	m/min = rpm × reed width (m)	600 × 1.664 = 999 m/min
Size add-on	(sized dry weight − unsized dry weight) ÷ unsized dry weight × 100	11% on 118.3 kg of warp = 13.0 kg of size

Two constants carry the unit conversions. 39.37 is inches per metre, so PPI × 39.37 is picks per metre. 23.25 is 590.5 ÷ 25.4: a count in Ne is 590.5 ÷ Ne tex, and there are 25.4 mm in an inch. Linear density in tex (grams per 1000 m) is used for every yarn quantity on this page; the yarn spinning formulas page explains count systems and their conversions, and the count converter turns 40s Ne into 14.76 tex.

Ends, picks and the reed

Ends per inch (EPI) and picks per inch (PPI) are counted on the cloth, as set out in ISO 7211-2. The total number of warp ends is EPI × cloth width, plus the selvedge ends, which are usually set closer and counted separately. On the loom the warp is wider than the finished cloth, because the weft bends over and under the warp and pulls it in by the weft crimp. So the warp must be spread in the reed over reed width = cloth width × (1 + weft crimp).

The reed count says how many dents (spaces between wires) the reed has. Divide the ends in each inch of reed by the ends drawn through each dent: dents per inch = total ends ÷ (reed width × ends per dent). Regional conventions differ: the Stockport count is the number of dents in 2 inches, and the metric count is dents per 10 cm (dents per inch × 3.937).

Example fabric	Working	Result
Total ends	120 × 63	7,560
Reed width	63 × 1.04	65.52 inches (1.664 m)
Ends per inch in the reed	7,560 ÷ 65.52	115.4, fewer than the 120 in the cloth because the weft pulls the cloth in
Dents per inch at 2 ends per dent	115.4 ÷ 2	57.7
Stockport count	57.7 × 2	115
Metric count	57.7 × 3.937	227 dents per 10 cm
Dents used	7,560 ÷ 2	3,780

Calculator: reed count and reed width, set to this fabric.

Fabric weight (GSM) from the construction

A square metre of cloth holds the warp ends crossing it and the picks crossing it, each longer than the cloth by its crimp. A warp of n ends per cm in yarn of T tex, crimped by c, weighs n × 100 × (1 + c) × T ÷ 1000 grams per square metre. Written with EPI and Ne this becomes 23.25 × EPI × (1 + c) ÷ Ne, and the weft is added the same way: GSM = 23.25 × [EPI × (1 + warp crimp) ÷ warp Ne + PPI × (1 + weft crimp) ÷ weft Ne].

Example fabric	Working	Result
Warp contribution	23.25 × 120 × 1.06 ÷ 40	73.9 g/m²
Weft contribution	23.25 × 80 × 1.04 ÷ 40	48.4 g/m²
Fabric weight	73.9 + 48.4	122.3 g/m²
Weight per running metre	122.3 × 1.600 m	195.7 g

Leaving the crimp out gives 116.2 g/m², 5% light. This is the loom-state weight; desizing, scouring, shrinkage and finishes all change it, which is why finished fabric is weighed rather than calculated (ISO 3801, ASTM D3776). Calculators: fabric GSM from construction, set to this fabric, target GSM solver (the construction for a weight you need) and fabric reverse engineering (the construction of a sample in hand).

Cover factor

Cover factor says how much of the cloth area the threads cover. Peirce took the diameter of a cotton yarn as about 1 ÷ (28 × √Ne) inches, so with threads per inch n the warp or weft cover factor is K = n ÷ √Ne, and K = 28 means neighbouring threads touch. Warp and weft overlap where they cross, so the cloth cover factor counts the overlap once: Kc = K1 + K2 − K1 × K2 ÷ 28. Fractional cover is Kc ÷ 28. The constant 28 assumes cotton-like yarn packing, so compare cover factors between fabrics of similar fibre.

Example: K1 = 120 ÷ √40 = 18.97 and K2 = 80 ÷ √40 = 12.65, so Kc = 18.97 + 12.65 − (18.97 × 12.65 ÷ 28) = 23.05, which covers 82% of the area. Calculator: cover factor, set to this fabric.

Crimp and take-up

Crimp is the extra length a yarn has because it bends round the threads it crosses. Unravel a yarn from a marked length of cloth, straighten it under a standard tension and measure it (ISO 7211-3): crimp % = (straightened yarn length − fabric length) ÷ fabric length × 100. Take-up states the same difference against the yarn length instead: take-up % = (straightened yarn length − fabric length) ÷ straightened yarn length × 100. A yarn of 106 cm from 100 cm of cloth has 6% crimp and 5.66% take-up. Warp crimp sets how much warp goes on the beam for each metre of cloth; weft crimp sets the reed width.

Calculator: crimp percentage and take-up.

Yarn requirement: warp and weft

Every end runs the length of the cloth plus its crimp, so warp weight in kg = total ends × fabric length (m) × (1 + warp crimp) × tex ÷ 1,000,000. In English units the same is pounds = ends × length (yards) × (1 + warp crimp) ÷ (840 × Ne). Every pick runs the width of the warp in the reed, so weft weight in kg = picks per metre × fabric length (m) × reed width (m) × tex ÷ 1,000,000, where picks per metre = PPI × 39.37. Add allowances on top for preparation waste, loom waste and the selvedge trim of shuttleless looms.

Example fabric, 1,000 metres	Working	Result
Warp	7,560 × 1,000 × 1.06 × 14.76 ÷ 1,000,000	118.3 kg
Warp with 3% preparation waste	118.3 × 1.03	121.8 kg
Weft	80 × 39.37 × 1,000 × 1.664 × 14.76 ÷ 1,000,000	77.4 kg
Check against the fabric weight	195.7 g per running metre × 1,000 m	195.7 kg = 118.3 + 77.4
Cloth from 1,000 kg of yarn	1,000,000 g ÷ 195.7 g per metre	about 5,110 m, before waste

Calculators: warp yarn requirement, set to this fabric, and the fabric GSM calculator, which also gives warp and weft kilograms per 100 metres. If the count of a sample’s yarn is not known, ISO 7211-5 describes measuring the linear density of yarn removed from the fabric (the site’s record).

Warping and sizing

• Direct (beam) warping. Number of warper’s beams = total ends ÷ creel capacity, rounded up, with the ends shared equally between them. 7,560 ends from a 640-position creel need 11.8, so 12 beams of 630 ends, which are combined at sizing.
• Sectional warping. Number of sections = total ends ÷ ends per section. 7,560 ends at 504 ends per section is 15 sections. See the sectional warping band calculator.
• Warp length on the weaver’s beam. Fabric length × (1 + warp crimp), plus the loom allowance for tying in and gaiting. For 1,000 m of cloth at 6% crimp that is 1,060 m plus the allowance.
• Size add-on. Size add-on (pick-up) % = (sized dry weight − unsized dry weight) ÷ unsized dry weight × 100, and size solids needed = warp weight × add-on ÷ 100: 11% on 118.3 kg of warp is 13.0 kg of size solids. Add-on is roughly wet pick-up % × solution concentration % ÷ 100, so a warp that takes up its own weight of an 11% solution carries about 11% size. See the size pick-up calculator and the wet pick-up and drying calculator.

Loom production and efficiency

A single-width loom inserts one pick for every turn of its main shaft, so loom speed in rpm is picks per minute. Divide the picks inserted in an hour by the picks in a metre of cloth: production in metres per hour = rpm × 60 × efficiency ÷ (PPI × 39.37), or ÷ (picks per cm × 100). Multiply by cloth width for square metres. Loom efficiency is the share of the available time the loom actually weaves, read from the pick counter: efficiency = picks inserted ÷ (rpm × minutes in the period) × 100. Weft insertion rate, in metres of weft per minute, is rpm × reed width (m); it compares looms of different widths, since a wide loom at a lower speed can lay more weft than a narrow one running faster.

Example fabric at 600 rpm and 85% efficiency	Working	Result
Picks per hour	600 × 60 × 0.85	30,600
Metres per hour	30,600 ÷ (80 × 39.37)	9.72 m
Per 8-hour shift and per day	9.72 × 8; 9.72 × 24	77.7 m; 233 m
Square metres per hour	9.72 × 1.600	15.5 m²
Efficiency from a pick counter reading 260,000 in a shift	260,000 ÷ (600 × 480) × 100	90.3%
Weft insertion rate	600 × 1.664	999 m/min

Calculators: loom production, set to this fabric and loom stops per 100,000 picks (CMPX).

Questions

What is the formula for loom production?

Production in metres per hour = loom rpm × 60 × efficiency ÷ (PPI × 39.37). A loom at 600 rpm and 85% efficiency weaving 80 picks per inch makes 9.72 metres an hour, or 77.7 metres in an 8-hour shift.

How do you calculate reed count?

Find the reed width (cloth width × (1 + weft crimp)), then dents per inch = total ends ÷ (reed width × ends per dent). Double it for the Stockport count, which counts dents in 2 inches. 7,560 ends over 65.52 inches at 2 ends per dent is 57.7 dents per inch, or 115 Stockport.

How do you calculate GSM from EPI, PPI and yarn count?

GSM = 23.25 × [EPI × (1 + warp crimp) ÷ warp Ne + PPI × (1 + weft crimp) ÷ weft Ne]. For 120 × 80 in 40s × 40s with 6% and 4% crimp that is 122.3 g/m² in loom state.

How much warp and weft yarn is needed for a fabric?

Warp kg = ends × fabric length (m) × (1 + warp crimp) × tex ÷ 1,000,000; weft kg = PPI × 39.37 × fabric length (m) × reed width (m) × tex ÷ 1,000,000. Then add waste allowances. The example fabric needs 118.3 kg of warp and 77.4 kg of weft for 1,000 metres.

What does a cover factor of 28 mean?

That the threads in that direction touch, leaving no gap between them. In Peirce’s system a cotton yarn is about 1 ÷ (28 × √Ne) inches thick, so threads per inch ÷ √Ne reaches 28 when they lie side by side. A cloth cover factor of 23 covers 82% of the area.

What is the difference between crimp and take-up?

Both measure the same extra yarn length. Crimp divides it by the fabric length, take-up by the yarn length. 106 cm of yarn in 100 cm of cloth is 6% crimp and 5.66% take-up.

What is weft insertion rate?

The metres of weft a loom lays per minute: rpm × reed width in metres. It is how looms of different widths are compared. A 1.664 m warp at 600 rpm is 999 m/min.

Sources and corrections

• Construction and its measurement: the site’s records for ISO 7211-2 (threads per unit length), ISO 7211-3 (crimp), ISO 7211-5 (linear density of yarn removed from fabric) and ISO 3801 (mass per unit length and per unit area); ASTM D3776 (mass per unit area); and ISO 1144 (the tex system).
• Cover factor: F. T. Peirce, “The geometry of cloth structure”, Journal of the Textile Institute 28 (1937), T45–T96.
• Preparation and loom calculations: S. Adanur, Handbook of Weaving (CRC Press); P. R. Lord and M. H. Mohamed, Weaving: Conversion of Yarn to Fabric (Woodhead Publishing).
• Units: NIST, Special Publication 811, for the exact inch, yard and pound behind 39.37, 23.25 and 840.

Change log. 8 October 2026: rewritten in full, with one example fabric worked through every formula so the results check each other. Corrected from the earlier version of this page: a yarn of 840 yards per pound is 1s Ne, not 840 (Ne counts 840-yard hanks per pound); denier is grams per 9000 metres, so 0.5 g per km is 4.5 denier, not 4,500; tex ÷ 1000 gives grams per metre, not kilograms per metre; weft weight comes from picks per unit length × fabric length × reed width, not from a total pick count × width; a single-width loom inserts one pick per revolution, so rpm is picks per minute; the “maximum EPI” example, which gave 50,000 ends per inch, is withdrawn in favour of Peirce’s cover factor; and the formulas, which had been displaying with raw formatting code beside them, are now plain text. The worked examples use the site’s calculators as they stood on this date.

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